In the field of temperature compensation for the light source unit of scanning optical systems, art has been proposed for compensating for the spatial variation between a laser light source 22 and a condensing lens 23 caused by temperature fluctuation by combining a plurality of condensing lens element holding member 21, as shown in FIG. 5, pursuant, for example, to the disclosure of Japanese Laid-Open Patent No.4-107518. Reference number 24 in the same drawing refers to a concave lens, and reference number 25 refers to a lens holder.
Japanese Laid-Open Patent Application No. 61-162014 discloses art for compensating temperature fluctuation caused by the oscillation wavelength of a semiconductor laser by changing the length of the light source support member. Furthermore, when the condensing lens is constructed as a single element lens, the focal length of the light source changes due to change in the oscillation wavelength of the laser induced by a change in temperature, and a method for compensating for the shift of the image forming point on the scanned medium caused by this variation is proposed in Japanese Laid-Open Patent Application No. 61-162014, which discloses a method for selecting the linear expansion coefficient of the support members so that the amount of change in the space between the condensing lens and the light emission point induced by thermal deformation of the light source support member is equal to the amount of change in the focal length.
In the construction disclosed in Japanese Laid-Open Patent No. 4-107518, however, the temperature compensation is applied to the space between the laser light source and the condensing lens, and the focal length of the condensing lens is not changed. In practice, the focal length is changed by a change in the refraction index caused by wavelength and temperature, and although it is possible to suppress this change in focal length by using a plurality of materials for the condensing lens elements, such a solution is disadvantageous from the perspective of increased cost of components and assembly due to the increased number of lens elements.
In the construction disclosed in Japanese Laid-Open Patent Application No. 61-162014, thermal expansion and change of the refractive index of the collimator lens due to temperature, and the influence of temperature on the f.theta. lens are not considered. Since the change in focal length is particularly great in the case of visible light, the support member must have a large linear expansion coefficient, which narrows the breadth of material selection, and the method only allows for changing the composition ratio of glass fibers in resin. When the support member comprises a member having a single type of linear expansion coefficient, the aforesaid composition ratio must be finely adjusted, thereby making it disadvantageously difficult to obtain resin materials inexpensively.
When the beam diameter is reduced by reducing the oscillation wavelength of the laser light and a single element collimator lens is used to reduce cost, the e reflective index of the material changed greatly due to the change in wavelength caused by the reduction of the oscillation wavelength, so as to disadvantageously be unbeneficial relative to temperature change.